1. Field of the Invention
The present invention relates to a taking optical system having a hand-shake correction function. More particularly, the present invention relates to a taking optical system that has a hand-shake correction function for reducing an image blur such as resulting from a hand shake or other vibration of a camera that may occur when shooting is performed with the camera held by hand, and that is suitable especially for use as a telephoto-oriented zoom lens system.
2. Description of the Prior Art
Conventionally, in a taking optical system having a hand-shake correction function, an image blur such as resulting from a hand shake or other vibration of the optical system is corrected either by the use of part of the lens elements constituting the taking optical system or by the use of an optical element other than lens elements that is provided in the taking optical system. In either case, it is essential that a taking optical system having a hand-shake correction function offer satisfactory optical performance not only in a normal state, in which hand-shake correction is not functioning, but also in a hand-shake correction state, in which hand-shake correction is functioning.
The former hand-shake correction method is adopted, for example, in the telephoto-oriented zoom lens systems having a hand-shake correction function which are proposed in Japanese Laid-open Patent Applications Nos. H5-232410, H5-224160, and H6-337375. In these zoom lens systems, hand-shake correction is achieved by decentering one of the zoom lens units constituting the zoom lens system or by decentering part of the lens elements constituting the zoom lens units in a direction perpendicular to the optical axis. Specifically, in the zoom lens systems proposed in Japanese Laid-open Patent Applications Nos. H5-232410 and H5-224160, a lens unit that is composed of more or less three lens elements is decentered for hand-shake correction in order to minimize aberrations that occur when the lens unit is decentered; in the zoom lens system proposed in Japanese Laid-open Patent Application No. H6-337375, a lens element that is composed of a doublet lens element is decentered for hand-shake correction in order to minimize chromatic aberrations that occur when the lens element is decentered.
The latter hand-shake correction method is adopted, for example, in the taking optical system having a hand-shake correction function which is proposed in Japanese Laid-open Patent Application No. H6-230446. In this taking optical system, an optical element, such as a prism, that acts as an optical wedge is disposed at the front or rear end of the taking optical system so that hand-shake correction is achieved through the wedge effect that is caused by such an optical element. Specifically, in the taking optical system proposed in Japanese Laid-open Patent Application No. H6-230446, two wedge prisms having diffracting optical surfaces are disposed at the rear end of the taking optical system so that axial lateral chromatic aberrations are suppressed by these diffracting optical surfaces.
However, in taking optical systems having a hand-shake correction function which adopt the former hand-shake correction method, since hand-shake correction is achieved by decentering part of the constituent lens elements, it is difficult to maintain satisfactory optical performance when those lens elements are decentered. In particular, in zoom lens systems, which need to offer satisfactory optical performance over the entire zoom range, it is extremely difficult to maintain satisfactory optical performance over the entire zoom range from the wide-angle end to the telephoto end when part of the constituent lens elements are decentered.
Specifically, in a construction such as those of the above-mentioned telephoto-oriented zoom lens systems proposed in Japanese Laid-open Patent Applications Nos. H5-232410 and H5-224160, where hand-shake correction is achieved by decentering a plurality of lens elements, the greater the number of lens elements included in the decentered lens unit, the larger and heavier that lens unit becomes. This is undesirable because, although decentering of the lens unit needs to be performed in real time as soon as an instantaneous image blur (such as resulting from a hand shake or other vibration of the optical system) is detected, it is difficult to decenter a large and heavy lens unit quickly enough, and also because it is necessary to use a drive system having accordingly large power.
Moreover, even in a construction such as that of the above-mentioned telephoto-oriented zoom lens system proposed in Japanese Laid-open Patent Applications No. H6-337375, where hand-shake correction is achieved by decentering a doublet lens element, it is inevitable that the decentered doublet lens element has approximately double the axial thickness of a single lens element. This is undesirable because it is difficult to decenter such a lens element quickly enough, and also because it is necessary to use a drive system having accordingly large power. Furthermore, in a construction of this type, where the number of decentered lens elements is reduced to a minimum, it is necessary to weaken the power of the decentered lens element in order to obtain satisfactory optical performance during hand-shake correction. Weakening of the power of the decentered lens element leads to an increase in the decentering amount by which the lens element needs to be decentered for a unit hand-shake angle. This means that the lens element needs to be decentered more quickly, and accordingly that its drive system needs to bear an increased load.
On the other hand, in taking optical systems having a hand-shake correction function which adopt the latter hand-shake correction method, it is necessary to secure a space for disposing a prism at the front or rear end of the taking optical system, and therefore it is inevitable that the taking optical system as a whole becomes accordingly larger. For example, in the above-mentioned taking optical system proposed in Japanese Laid-open Patent Application No. H6-230446, it is necessary to secure a space for disposing two prisms at the rear end of the taking optical system. This makes the taking optical system as a whole larger by the extra space secured for the prisms. Moreover, in a taking optical system of this type, it is necessary to use two such hand-shake correction blocks to correct an image blur properly, because, in general, an image blur needs to be corrected in at least two directions. Such a construction, however, is impractical.